When the electric motor in the motor control arrangement is operated with a supply voltage applied to the first and the second supply voltage terminal a duty cycle impressed on the switching transistor by the pulse width control stage dictates the speed of the electric motor (Pulse Width Modulation), a voltage generated by the electric motor (EMF voltage) being directly proportional to the speed. A simple speed control is obtained when the pulse width control stage is controlled by means of the motor voltage measurement stage in response to the motor voltage. However, since the motor voltage depends both on the voltage generated by the electric motor (EMF voltage) and on a voltage generated by the motor current an inaccurate speed control is obtained, the voltage generated by the motor current being caused by a motor resistance of the electric motor. An improved speed control is achieved when the pulse width control stage is controlled not only by means of the motor voltage measurement stage in response to the motor voltage but also by means of the motor current measurement stage in response to the motor current. From the control in response to the motor voltage and the motor current the pulse width control stage can derive an indication of the difference between the motor voltage and the voltage generated by the motor current, which difference is a measure of the voltage generated by the electric motor (EMF voltage) and of the speed of the electric motor.
A motor control arrangement as mentioned above, which arrangement comprises a control circuit including the motor voltage measurement stage, is known inter alia from the chip which is commercially available from Philips Electronics N.V. under the type number TEA 1019. In the relevant chip the second connection terminal is coupled to an input of a current mirror by means of a first resistor and the first supply voltage terminal is coupled to an output of the current mirror by means of a second resistor. As the input of the current mirror receives a current which is related both to the motor voltage and to a voltage appearing across the first resistor and as the output of the current mirror receives a current which is related to a voltage appearing across the second resistor a current related to the motor voltage can be taken from the output of the current mirror. A disadvantage of the present motor voltage measurement stage is that the first and the second resistor cannot be integrated, which leads to a complex and expensive manufacture, an inaccurate measure of the motor voltage, and an inaccurate speed control.
A motor control arrangement as mentioned above, which arrangement comprises a control circuit including the motor current measurement stage, is known inter alia from the Japanese Patent Application bearing the Application number 59-7544. According to the relevant Patent Application a voltage generated across the measurement resistor is used in order to obtain a measure of the motor current. The voltage generated across the measurement resistor is applied to a non-inverting input of a comparator, which has an inverting input coupled to the first supply voltage terminal by means of a voltage source and which has an output coupled to the pulse width control stage. As a result, the duty cycle of the pulse width control stage is controlled in dependence upon the difference between the voltage generated across the measurement resistor and a voltage supplied by the voltage source, which difference is a measure of the motor current. A disadvantage of the present motor current measurement stage is that the voltage supplied by the voltage source must be adapted to the supply voltage, which is prohibitive of a simple use of the motor current measurement stage. A further consequence of the described relation between the voltage supplied by the voltage source and the supply voltage is that a variation of the supply voltage results in a variation at the non-inverting input of the comparator and in a variation of the duty cycle, leading to an inaccurate speed control.